High temperature protective coating for metals



United States Patent HIGH TEMPERATURE PROTECTIVE COATING FOR METALSEdward B. Schneider, Canoga Park, and Everett G. Stevens, El Segundo,Califl, assignors to North American Aviation, Inc.

No Drawing. Application March 25, 1958 Serial No. 723,634

16 Claims. (Cl. 148 13.1)

This invention relates to a high temperature protective coatingcomposition for metals. More particularly this invention relates to acoating composition which protects metal surfaces from oxidative andcorrosive deterioration at elevated temperatures.

At temperatures required to heat treat metals such as titanium and itsalloys, the metal is subject to gaseous contamination. Thiscontamination results in a brittle layer on the surface and renders thematerial unsuitable for forming processes and structural use. Thisbrittle layer must be removed before the material can be furtherfabricated or used as a structural member. Sandblasting, grinding and/oracid etching are among the methods which have been employed to removethis contaminated layer. Sandblasting can result in uneven metal loss;grinding produces sharp notches (stress risers); and acid etching oftenraises the hydrogen content above a tolerable amount. These proceduresin addition to destroying expensive metal are time consumingand giverise to many production problems. Controlled atmospherefurnaces orencapsulating the titanium sheet in a steel bag are among the methodsemployed to prevent this contamination. These methods are inconvenient,complex, and in some cases costly. Therefore a more simplified andeconomical method for the high temperature processing of titanium isneeded.

It is therefore an object of this invention to provide a coating whichwill protect metals at high temperatures. It is also an object of thisinvention to provide a coating which will protect metals during heattreatment at elevated temperatures. Another object is to provide acoating which will protect titanium during heat treatment. It islikewise an object ofthis invention to provide a coating compositionwhich will prevent gaseous;

contamination of titanium and its alloys during treatment attemperatures in excess of 1100 F. Still another object of this inventionis to provide a coating composition which will substantially reducewarpage of titanium and titanium alloy sheet stock when water quenchedfrom temperatures in excess of 1100 F. It is also an object to provide aprocess for heat treating of metals so as to prevent contamination ofthe surface by the gases of the surrounding atmosphere. It is like- Wisean object of this invention to provide coated metal articles which arenot subject to corrosive attack by atmospheric elements. Still otherobjects will be apparent from the discussion which follows.

The above and other objects of this invention are ac complished byproviding a high temperature protective coatingcomposition for metalcomprising 100 parts by weight of a ceramic material, from about 10 toabout 200' parts by weight aluminum flakes, from about 1 to about 100parts by weightof a bentonite clay, and an amount of diluent sufficientto give the composition a spreadable consistency. When a coatingcomposition of this nature is applied to the surface of a metal article,the article may then be subjected to heat treatment at elevatedtemperatures without contamination of gaseous 2,898,253 Patented Aug. 4,19 59 P CC . 2" elements of the surrounding atmosphere. An example isthe heat treatment of titanium covered with a coat-j ing of thiscomposition at temperatures in excess of 1100" F. There is noembrittlement of the titanium a r-I ticle. When, however, titanium isheat treated without a protective coating the surface oxidizes andbecomes brittle which renders the material unsuitable for formingprocesses and structural use. H v p The ceramic material that isemployed in the composition of this invention is a ceramic vitreousfrit, sometimes also referred to as a porcelain enamel frit or aceramically maturable vitreous enamel frit composition. An example of afrit of this type is Percent by wt.

Quartz 34.1 Borax 21.8 Feldspar 19.9 Soda ash 11.2 Fluorspar Sodiumnitrate 4.9 Lithium manganite 2.1

hereinafter referred to as frit A, which is found to be satisfactory. AnNBS ceramic coating frit designated as No. 332, hereinafter referred toas frit B is also found to give satisfactory results. It has thefollowing 00111 Another vitreous ceramic composition which is found togive satisfactory results; is a frit, hereinafter referred to as frit C,whose composition is V p Percent by wt.

SiO 37.50 Alumina 1.00 "Boric acid 6.50 Calcium oxide 3.50 f Bariumoxide 44.00 Zinc oxide V 5.00 Zirconium oxide 2.50

While the above are examples of various frits, composition of thisinvention is not limitedito the'use of these specific frits. Any ceramicfrit which is niaturable into a vitreous compositionmay be used.

Best results are obtained if the frit, prior to use, put through a ballmill and the particle sizereduced so that it will pass through about a70-mesh screem However, frit of particle, size which pass through ascreen of about 20 to about mesh can also beused in the composition ofthis invention. 'One method accomplishing this is; to subject the an to.the action of a dry stage pebble mill for a. period of. about 2.4.-hours. x p

The aluminum that,v is used in the'composition: of this. invention is inthe form of aluminu'rii flakes; Aluminum is reduced to alumiuum fiakes.by subjecting, alumi num powder to the action of a ball mill which,reduces the powder to fine flake-like particles. One form in which thealuminum may be employed is as an al'umi num paste which is prepared bymixing aluminum pigment with a liquid thinner such as toluene or withpetroleum fraction hydrocarbons of the naphtha group, together with aleafing agent such as stearicor palmitie acids and subjected to theaction of a ball mill. When used in this form the aluminum solidscontent of the paste is usually from about 50 to about 75 weightpercent. The composition of the liquid thinner and leafing agent isimmaterial since these hydrocarbon compounds are burned off before thefusion temperature to which the coating is subjected prior to and duringthe heat treatment of the coated metal. An example of a suitablealuminum paste is one containing 65 weight percent aluminum flakes in anaphtha solvent. The amount of coarse particles is such that not morethan 0.5 weight percent are retained on a 100 mesh screen. Aluminumpastes in which not more than from about 0.1 to about 1.0 weight percentare retained on a 325 mesh screen are also satisfactory.

' 'I'he bentonite clay composition which is used in the coating of thisinvention is a well-known clay composition which swells upon theabsorption of water and organic diluents and has strong absorbingproperties. Bentonite is sodium montmorillonite or sometimes referred toas sodium bentonite. The bentonite may be used either in its originalclay form or in the form of an organic substituted ammonium bentonitecomposition which is the product of the interaction between sodiumbentonite and an organic ammonium compound. Thus, the term bentoniteclay composition embraces boththe bentonite in its original clay formand also the organic ammonium bentonite. The organic ammonium bentonitecompounds may be represented by the general formula:

wherein the N is nitrogen; Y is a clay substituent such as bentonitewhich includes montmorillonite, a component of bentonite; the R groupsmay be the same or different and represent hydrocarbon groups which havereplaced hydrogen atoms attached to the nitrogen atom and are selectedfrom the class consisting of alkyl, cycloalkyl, alkenyl, cyclo-alkenyl,aryl, arylkyl and alkaryl groups having from 1 to about 20 carbon atoms.

Non-limited examples of mono-hydrocarbon substituted ammonium bentonitecompounds are 2-ethylhexylammonium bentonite, hexadecylammoniummontmorillonite, dodecylpyridinium bentonite, octadecylammoniumbentonite, etc. Examples of dihydrocarbon substituted ammonium claycompounds are di-Z-ethylhexylammonium bentonite, methylnaphthylammoniumbentonite, didodecylammonium bentonite, etc. Examples of tri-hydrocarbonsubstituted ammonium bentonite compounds are trioctadecylammoniumbentonite, methylcyclohexyldodecylammonium bentonite,dimethyloctadecyiammonium montmorillonite, etc. Non-limiting examples oftetra-hydrocarbon-substituted ammonium bentonite compounds aretetramethylammonium bentonite, dimethyldiethylammonium montmorillonite,dimethyldioctadecylammonium bentonite, methyltributylammoniummontmorillonite, dimethyldodecylbenzylammonium bentonite,tetrabutylammonium bentonite, methyltrieicosylammonium bentonite,diethyldiphenylammonium bentonite, trimethylnaphthylammoniummontmorillonite, dibutyldicyclohexylammonium bentonite,diethyldiisohexenylammonium bentonite, dipropenyldioctadecylammoniumbentonite, dimethyldipropenylammonium bentonite,dimethyldi(2,4-di-heptylphenyl)ammonium bentonite.

Other organic ammonium clay compounds which are employed in the processof this invention are compounds in which the organic substituent on thenitrogen atom is an ether substituent-containing hydrocarbon, i.e., thesubstituent contains one or more -C-OC groups therein, as for example,ethylhexoxypropylammonium bentonite,octylphenoxyethoxyethyldimethylammonium bentonite, etc. Other ammoniumclay compounds in- .clude rosinammonium bentonite, pyridinium bentonite,

etc.

The amine:clay ratio in the organic ammonium clay compound can have avalue of 50 to 200 milliequiv. amine per 100 grams of clay. When anammonium bentonite compound is employed, it is preferred to use a claycomposition in which the amine:clay ratio is 75-ll5 milliequiv. amineper 100 grams of clay in order to obtain better protection upon heattreating of metals coated with a composition of the clay compound.Especially preferred is a clay compound in which the amine:clay ratio issubstantially 80 milliequiv. amine per 100 grams of clay, thetheoretical base exchange value for the combination.

The above hydrocarbon substituted ammonium bentonite compounds are madeby the process substantially as described in Kolloid-Zeitschrift, 137.Band, Heft 1, Seite 40 (1954), and in US. 2,531,427. The hydrocarbonsubstituted ammonium bentonite is in a powdered form of a particle sizesubstantially such that it will pass through a 200 mesh screen. However,particle sizes from about 0.002 to about 100 microns in diameter aresuitable.

A coating composition containing a ceramic material, aluminum, and abentonite clay composition affords pro tection to metal surfaces fromdeterioration due to attack by gaseous components during heat treatmentat elevated temperatures. However, it has been found that occasionallythe surface of a heat treated coated article has become slightlycontaminated due to the action of oxygen and other components in theair, indicating that flaws or weak points may develop in the coating. Ithas also been found that the addition of nickel powder to the coatingcomposition eliminates any failure in the protection rendered by thecoating at all temperatures ranging from i about 1100" F. to about 1850F. This is especially true in the case of heat treatment of titanium.Therefore, a coating composition as described above which contains inaddition from about 1 to about parts by weight of nickel powderconstitutes a preferred embodiment of this invention. The particle sizeof the nickel powder that is used is such that the powder will passthrough a screen having a mesh of from about 200 to about 600 mesh. Forexample, good results are obtained when the particle size of the nickelpowder is such that it will pass through a 325 mesh screen.

While the coating composition may be used in the form described above,it is advantageous to add a binding composition which will impart abetter spreadable consistency for application purposes. The presence ofa binder is also found to give added protection to the coated surfacewhile it is being heated up to the fusion temperature of the coating. Anembodiment of this invention is thereforea casting composition-asdescribed abovewhi'ch conrains in' addition from about 1' to about 200parts; by weight of a compatible organic binding composition based on100 parts by weight of ceramic material. The type of compositionemployed is not critical so that any compatible binding composition maybe used. It may be any polymeric, resinous, or plastic material whichwill serve to bind the solid particles in the coating together so as toform a continuous film on the surface of the metal to" which it isapplied. Hence, the binding composition can be composed of acrylateresins, polymeric epoxy resins, polyurethans, alkyd resins, copolymersof various resins, and the like. The preparation of these resins is wellknown to those skilled in the art and can be found in such textbooks asOrganic Chemistry, by Fieser and Fieser, published by D. C. Heath & 60.,Boston. Nonlimiting examples of acrylate resins which can be used in thecoating are methylmethacrylate, ethylrnethacrylate, n'propylmethyacryl'ate, diethylen'eglycolmethacrylate, methylethacrylate,etc. An example of an alkyd resin is the condensation product ofglycerol and phthalic anhydride. Other alkyd resins are well known tothose skilled in the art. An example of an epoxy resin is a glycidylpolyether resin obtained by the reaction upon heating of epichlorohydrinwith 2,2-bis( 4-hydroxyphenyl) propane in the presence of sodiumhydroxide asa catalyst. This produces a polymer in which the molecularunits having terminal epoxy groups. Other epoxy resins prepared by thereaction of epichlorohydrin with a polybyd' c c ol such as. l., .3- hvroxvp op ne re. well known in the art. An example of a polyurethan is adiisocyanate of propylene glycol of the general formula wherein n is anumber taken from the series 0, 1, 2, 3, and having an average molecularweight of substantially 2500.

The diluent used in the preparation of the coating of this invention isany compatible diluent including Water. This includes any of the wellknown diluents employed with resins and polymers" in the paint industry.Nonlimitin'g examples of diluents include lower aliphatic alcohols,lower aliphatic ketones', lower alkyl esters of lower aliphatic acids,and lower hydrocarbons such as benzene and lower alkyl substitutedbenzenes, all containing up to about 14-. carbon atoms. Non-limitingexamples of these diluents are acetone, methylethyl ketone, diethylketone, diisopropyl ketone, octyl hexyl ketone, methylacetate,butylacetate, octylacetate, methyl propionate, octylhexanoate, benzene,toluene, xylene, ethyl benzene, tern-butyl-benzene, e tc. p v V I Theresin may contain various customary compatible plasticizers, the natureand amounts of which are Well known to those skilled in the art and willnot be discussed in this writing.

The amount or diluent employed with the coating can vary froml to timesthe. amount of combined ceramic frit, aluminum bentonite and resincompounds in parts by: weight. The amount of diluent, can be adjusted tosuit the particular application, namely, brush application, spraying,dipping or other appropriate means for spreading the coating on thesurface. A ratio of combined solid components-to-diluent of 1:1 is foundto be satisfactory when' the coating is spread with a spatula on thesurface to be protected. For spray application, it is found that thecomposition is of the proper consistency when the ratio of solids todiluent is about 1:10. Still greater amounts of diluent maybe employedif desired, however, ordinarily amounts in excess of that which wouldgive a ratio of solidsto-diluent of 1:10 give. no additional advantageand only increase the amount of diluent that must be evaporated from thecoating.

Small amounts of low boiling organic compounds having from about 2 toabout. 8 carbon atoms. such as from about 0.1 to about 5. weight percentethyl alcohol based on the weight ofth dilu nt-may be added to aid inthe subsequent evaporation of the diluent fromthe coating. The loweralkyl alcohols can be employed as diluent either alone or in combinationwith: other diluents. These alcohols can have from about lto aboutscarbon atoms and includes such compounds as methyl alcohol, ethylalcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, amyl alcohol,octyl alcohol, phenylethyl alcohol, etc.

In' addition to a; process for heat treating metals; an embodiment ofthis invention provides coated metal articles in which the coatingprotects the surface from oxidation at ambient atmospheric conditions aswell as at cle v'ated' temperatures. In this manner, metal articleswhich are to be heat treated may be protected from oxidation for longperiods of time prior to the actual treating of the articles. Therefore,an embodiment ofthis invert tion includes a metal article having acoating consisting of a composition which" is more particularlydescribed elsewhere in this writing. For example, an embodiment of thisinvention comprises a metallic article having a surface coatingconsisting essentially of a mixture of a ceramic material, aluminum anda bentonite clay composition. A preferred embodiment of this inventionis a. metal article having a surface coating as described abovecontaining nickel powder in addition thereto. Another embodiment of thisinvention is a metallic article havinga surfacecoating consistingessentially of a ceramic material, aluminum, a bentonite claycomposition, a cornpatible organic binding composition, with or withoutnickel powder. 1 V

When the coated metallic article has been initially heated to atemperature sufficient to decompose the orgame material in thecomposition to'form a film of noncombustible material on the surface ofthe article, it is then heated to a temperature suflicient' tof fuse theThereafter the coated article is heat tr'eated at a prodeter'rnin'edtemperature. I

The surface coating upon fusion consists essentially of a fused mixtureof ceramic material, aluminum, bentonitei clay composition, with orwithout the presence of nickel. The proportions of these variouscomponents of the fused coating depends on the amount that wasoriginally added in the preparation of the coating composition asdescribed elsewhere in this Writing. A metallic article containingsuch afused surface, coating constitutes an embodiment of this inventionsince. it provides a metal; li'c article having a protected surface. Thearticle can be put into service in certain applications where hightemperature surface protection is required without removal of thecoating. For example, such a coated tita nium article will be protectedfrom'oxygen contamination after the heat treatmentv cycle attemperaturesupv to the heat treating temperature, and higher. p I o 'While theamount of aluminum that is used per 10.0 parts by weight of ceramicmaterialcan vary from about 1 t about 1mm by we t... it. s fe n h t posibility of failure of the protection afiorded by the coat ing can begreatly reduced when the amount of alumi: num is from about 50 to about150 parts by weight per parts of ceramic material. Therefore, a coatingcomposition containing an amount of aluminum Within the latterdesignated range constitutes a preferred embodi; ment of this invention.In like manner, it is found that a preferred amount of bentonite claycomposition varies from about 2 to about 50 parts by weight per 100parts of ceramic material. The preferred amountof nickel powder forbetter protection is from about 2 to about 50. parts by weight per 100partsof ceramic material.

It is also found that in order to insure cohesiveness'ofthe coatingmaterial upon application to the. surface of the metallic, article, itis advisable to employ from about 50 to about parts by weight of; anorganic binding composition per 100 parts of ceramic. material. All theabove ranges of components constitute a preferred em-: bodiment ofinvention.

An especially preferred composition which gives optimum protection tometallic articles both before and after fusion is one comprisingsubstantially 100 parts by weight of a ceramic material, substantially70 parts by weight aluminum flakes, substantially 10 parts by weightdimethyldidodecyl ammonium bentonite, substantially 10 parts by weightnickel powder, substantially 100 parts by weight methylmethacrylateresin and sufficient volatile diluent to give the composition aspreadable consistency. An example of diluent is about 220 parts byweight of methyl isobutyl ketone.

The general process for the preparation of the coating composition ofthis invention consists of mixing together the solid components, namelythe ceramic material, the aluminum flakes, the bentonite claycomposition, and the nickel powder if it is used, together with thebinding composition if employed and whatever diluent is used to give thecomposition a spreadable consistency. The mixing can be accomplished bystirring, tumbling, milling or any other appropriate means well known tothe art. One method of thoroughly mixing the components is to subjectthem to the action of a ball mill for a period of from about 4 to about16 hours. Alternatively, each component may be reduced to the properparticle size by grinding, comminuting, or other appropriate means suchas by the action of a ball mill. One method is to charge the ceramicmaterial, the bentonite clay composition, together with the nickelpowder and acrylic resin if the latter two are employed, and a smallamount of diluent to a ball mill and subject the components to thegrinding action of the mill for a period of from about 4 to about 24hours. The components are then removed and aluminum flakes added eitherin the dry or paste form and blended in by mixing, stirring, or otherappropriate means until each component is evenly dispersed throughoutthe entire composition. The following examples will more clearlyillustrate the composition. and process of this invention.

- EXAMPLE I EXAMPLE II The process of Example I is repeated with themodification that about 1 part of nickel is added to the ball mill.

EXAMPLE In The procedure of Example I is repeated with the modificationthat about 1 part of nickel powder together with about 1 part ofmethylmethacrylate resin is added.

EXAMPLE IV The procedure of Example III is repeated with themodification that a 50/50 mixture of ethyl alcohol and acetone issubstituted in place in the water as a diluent.

EXAMPLE V To a ball mill are added substantially 100 parts by weight ofthe ceramic frit C, substantially 70 parts by weight of aluminum flakes,substantially 10 parts by weight of dimethyl didodecyl ammoniumbentonite, substantially 10 parts by weight of nickel powder,substantially 30 parts by weight of methylmethacrylate resin, and about220 parts by weight of methyl isobutyl ketone. The contents aresubjected to the grinding action of a ball mill for a period of about 16hours. The contents are then removed and the ball mill washed with anadditional'amount of about 70 parts of methyl isobutyl ke tone and thecomposition with added washing is stirred until a uniform consistency isobtained.

EXAMPLE VI To a ball mill were added 50 parts by weight of ceramic fritC, 5 parts by weight of nickel powder, 5 parts by weight ofdimethyldidodecyl ammonium bentonite, about 60 parts by weight of analuminum pigment paste consisting of aluminum flakes in an n-heptanevehicle which contained about 5 weight percent stearic acid as a leafingagent with the aluminum flake content of the paste being about 65 weightpercent, about 15 parts by weight of methylmethacrylate resin, dissolvedin about 35 parts of methyl ethyl ketone and about 45 parts of methylisobutyl ketone. The components were subjected to the grinding action ofthe ball mill for a period of about 16 hours after which time thecomponents were removed and the mill washed out with an additional 15parts of methyl isobutyl ketone and the washing added to the removedcontents. The ground composition and added washings were stirred until acomposition of uniform consistency was obtained.

EXAMPLE VII The procedure of Example VI was repeated with themodification that the aluminum pigment paste was not added to the ballmill, but was added to the milled components after removal from the ballmill. The milled components together with the aluminum paste and addedwashing fluid were stirred until a composition of uniform consistencywas obtained.

The following table contains examples of still other coatingcompositions of this invention in which the components are present invarious proportions.

Table Parts by weight Composition No.

Frit A 100 100 1 Frit B 00 100 Frit O 100 Aluminum flakes 150 100 75 200Bentonite 50 25 Dimethyldidodecl ammonium bentonite 50 15 50 2 100 20 75100 200 50 Methyl isobutyl ketone. 100 2, 000 Toluene 200 50 5, 000 20Ethylacetate 100 n-Heptane 100 50 100 l Resin D is an alkyd resinobtained by the condensation of ethylene glycol with phthallicanhydride.

2 Resin E is an Epoxy resin obtained by heating together equimolarquantities of epichlorohydrin and 2,?Fbis(4-hydroxyphenyl) propane inthe presence of a sodium hydroxide catalyst, and having an epoxideequivalent; of about -210 grams of resin per gram equivalent of epoxideThe metal article which is to be heat treated is coated with thecomposition of this invention by spreading, brushing, or spraying so asto provide a coat having a thickness of from about 0.25 to about 20 mils(thousandths of an inch), preferred 0.5 to 6 mils. The coating is thenallowed to air dry for a short period of time of from about 1 to about20 minutes in order to allow the excess diluent to evaporate. Thecoating is sufficiently dry when it is firm to the touch and exhibits notackiness. The coated metal article is then ready for heat treatment. Inone embodiment of this invention the coated metal article is placed in afurnace originally at ambient temperature and then heatis applied tobring the temperature of the coated article up to a point at which theorganic material in the coating is decomposed and burned off, leaving ananhydrous film having no carbon, hydrogen or ammonium therein. Thisusually occurs at a temperature of substantially from about 500 F.'toabout 700 F. Following this more heat is applied sutfia cient tofuse thecoating and then additional heat is applied to bring the article to thepredetermined heattreating temperature of the metal in question. Themetal is heat treated at a temperature of substantially from about 1100F. to about 1850 F. for periods of time of from about 0.5 to about 4.0hours. In the case of alloys of titanium such as alloys containing 6weight percent aluminum and 4 weight percent vanadium, the article issubjected to heat treatment at a temperature of substantially 1700 F.for a period of about 30 minutes to'eifect what is known as solutiontreatment which consists of maintaining the alloy at this hightemperature so as to form a solid solution of aluminum and vanadium inthe titanium. After this heat treatment the titanium article isimmediately water quenched. Articles of other metals may be brought toambient temperatures by slow cooling in an airstream.

The film left on the metal article after heat treatment is readilyremoved by subjecting it to the action of aqueous mineral acid solutionssuch as aqueous solutions of hydrofluoric and nitric acids for a periodof from about 10 to about 30 minutes. When, however, the metal is heattreated without the coating composition of this invention on itssurface, the outside scale which is formed is much greater than thecoating composition thickness and cannot be removed by acid solutions.Instead, it must be sandblasted, vapor blasted, or machined off.

The following examples will more clearly illustrate the protectionafforded metallic articles during heat treatment. I

EXAMPLE VIII The coating composition of Example V was reduced with anadditional 150 parts by weight of methyl isobutyl ketone in order togive a consistency applicable for spray application. The composition wasthen sprayed onto a titanium article consisting of 90v Weight percenttitanium, 6 weight percent aluminum and 4 weight percent vanadium, afterthe surface had first been cleaned with methyl ethyl ketone solvent toremove any grease or foreign matter. applied to the surface with asubsequent air drying period of about 15 minutes. This provided acoating substantially 6 mils (0.006 inch) in thickness. The coatedtitanium article was then placed in an oven maintained at 600 F. for 30minutes during which time the organic material volatilized or oxidizedand decomposed leaving a film of combined ceramic frit aluminumbentlonite and nickel. The titanium specimen with the coating was thenplaced in a furnace maintained at a temperature of substantially 1700 F.for a period of about 30 minutes. The article was then removed from thefurnace and immediately quenched in water.

The surface film or scale on the coated titanium article' which had beenheat treated was found to be substantially 1 mil thick. Analysis or thesurface scale shows there is no hydrocarbon, nitrogen or water presentand that only aluminum oxide, aluminum silicate, silica, nickel oxideand the components of frit C make up the film indicating that the filmconsists mainly of the fused components of nickel, aluminum bentoniteclay and ceramic frit. The article was then immersed in an aqueoussolution containing 50 weight percent sodium hydroxide and 1 weightpercent sodium chromate maintained at 275 F. followed by an aqueous acidsolution containing 3 weight percent hydrofluoric acid and 35 weightpercent nitric acid for a period of substantially 10 minutes. Thisremoved the surface film leaving a shiny smooth surface.

EXAMPLE IX A commercially pure titanium metal article 1.0 inch x 10.0inches x 0.032 inch was covered with the composition of Example I bymeansof a brush to provide a surface film of approximately 20 mils inthickness. The

A total of two spray coats were I0 film was allowed to dry in the airfor about 45 minutes. The film-coated article was then placed in thefurnace maintained at 1400 F. and kept there for a period ofsubstantially 4 hours. The article was then removed from the furnace andimmediately quenched in water. The titanium article did not warp uponquenching.

A similar specimen of titanium treated as described in Example IXwithout the protection of a coating of this composition, warped uponquenching to a degree of 6 percent to 10 percent variation from flat.

EXAMPLE X A specimen of 6Al4V titanium containing 6 percent aluminum, 4percent vanadium, and measuring 36 inches by inches by .040 inch wascoated with the composition of Example V according to the proceduredescribed in Example VIII to form a film 4.0 mils thick. After airdrying and baking at 500 F. for a period of substantially 30 minutes atwhich temperature the organic material in the coating decomposed and thecoating formed a film on the surface of the article, the specimen washeat treated in an oven at 1700 F. for 30 minutes and then waterquenched immediately. The coating which fused at the heat treatingtemperature was removed by vapor blasting. The sheet was then heat agedat 1000 F. for 4 hours. Warpage was from 2.5 to 3.5 percent variationfrom flat.

Several specimens were prepared according to the procedure described inExample IX and subjected to various tests. A specimen treated accordingto Example IX with the protection of the coating of this composition wassubjected to a bend test. It was found that a radius of bend of 3.8T wasobtained with no failure in the specimen material. T is the thickness ofthe material and in this case was 0.040 inch. However, a specimen, whichhas been treated in like manner but without the coating of thiscomposition on its surface, cracked upon being bent to a bend radiusgreater than 10T.

A 6A1-4V titanium specimen which had been coated and treated asdescribed in Example IX was subjected to tensile tests. It was foundthat it had a transverse yield strength at 157,500 p.s.i., an ultimatetensile strength at 171,100 p.s.i. and an elongation of 7.8 percent,whereas a similar article treated in like manner, but without a coatingof this composition had a yield strength of 166,000 p.s.i., an ultimatestrength of 179,000. p.s.i., and an elongation of 2.7 percent. It isseen that there is an increase of 35 percent in elongation on the coatedspecimen.

EXAMPLE XI a An AllOAT titanium disk 24 inches in diameter and 0.187inch in thickness composed of titanium containing 5 weight percentaluminum and 2.5 weight percent of tin, was coated with a composition ofExample V to a thickness of 5 mils. The coating was dried in air for aperiod of 30 minutes and then subjected to a temperature of 700 F. for30 minutes. The coated titanium specimen was next annealed at l600 F.for a period of 30 minutes and then air cooled. The coating was thenremoved from the titanium specimen by subjecting it to an aqueouscaustic solution containing 50 weight percent of NaOH and 1 percent NaCrO maintained at 2751:5" F., followed by an aqueous acid solutioncontaining 3 weight percent hydrofluoric acid and 35 weight percentnitric acid, for a period of substantially 10 minutes. This removed thefilm leaving a shiny smooth surface. The specimen was then formed into a16 inch diameter hemisphere on a 7000 ton hydroform press. No flaws orcracks appeared in the titanium during this forming process.

A disk of titanium similar to that employed in Example XI which had beentreated without a coating composition, cracked severely during thepressing in the hydroform press and could not, be drawn out into ahemispherical shape. This indicates that the surface of the uncoatedspecimen was contaminated by oxygen and other gaseous impurities duringthe annealing cycle and this caused the specimen to fail upon beingsubjected to the forming process in the hydroform press.

A titanium specimen measuring 0.032 x 1.0 x 10.0 coated with thecomposition of Example VI to form a film 0.25 mil thick and subjected toheat treating temperature of 1850 F. for a period of 30 minutes showedacceptable bend and tensile strength characteristics. In like manner thespecimens heat treated while covered with coatings of Examples I-VII andwith the coatings shown in the table, exhibited satisfactory tensile andbend characteristics.

Specimens of steel and Inconel X when heat treated with a coatingcomposition as described in Examples I-VII or with one of thecompositions shown in the table, are found to be adequately protectedfrom oxidization and gaseous surface contamination during heattreatment.

Titanium articles are important in the manufacture of structural partsfor high-speed aircraft. It is vital that such titanium possess maximumtensile and yield strength. Therefore, contamination by oxygen or othergases in the atmosphere during annealing cannot be tolerated. Forexample, two of the 16 inch diameter spherical or dome-shaped articlesdescribed in Example XI are welded together to form a tank constructionused to house fuel on high-speed aircraft. It is seen that without theuse of applicants coating composition such a dome-shaped titanium membercannot be manufactured unless other inconvenient, complex, and costlymethods are employed to remove or prevent the contamination of thetitanium alloy as discussed hereinabove.

EXAMPLE XII A titanium billet measuring 6.0 x 6.0" x 18.0" is coveredwith the composition marked No. in the table, by means of brushapplication to a thickness of about 20 mils. The coating is air driedfor a period of 1 hour and then baked at 600 F. for a period of 15minutes. The billet is then heated to 1850 F. and squeezed down in aforging press to a 20 percent reduction in crosssection. The surfacefilm is then removed from the reduced billet by sandblasting to a smoothclear surface with a surface metal loss of approximately 0.002 inch.

When the experiment of Example XII is repeated Without a coating, anoxide scale 5 mils thick develops which on removal results in a surfacemetal loss of 0.005 to 0.010 inch.

In like manner a 4130 carbon steel specimen coated with composition No.4 of the table to a depth of 0.5 mil prevented scale formation on thesteel specimen while being normalized at 1800 F. for 30 minutes. Thespecimen with no composition to protect it developed a scale 0.006 inchthick which could not be removed by pickling and had to be sandblastedofi leaving an uneven or rough surface.

EXAMPLE XIII A specimen of 6A1-4V titanium, coated to a depth of 5 milswith composition No. 2 of the table by means of dipping, when subjectedto an aging treatment at 1100 F. for 4 hours develops a negligibleamount of scale on the surface which is readily removed upon immersionin an aqueous sodium hydroxide bath containing sodium chromate, followedby an acid bath.

When the heat aging process of Example XIII is repeated without the useof a coating of this invention, the scale formed is thicker than inExample XIII and cannot be removed by a sodium hydroxide-sodium chromatebath, but requires a more drastic technique such as sandblasting whichnot only is more costly and time consuming, but results in the loss ofan appreciable amount of metal from the surface.

Although the invention has been described and illustrated in detail, itis to be understood that the same is by way of illustration and exampleonly and is not to be taken by way of limitation, the spirit and scopeof this invention being limited only by the terms of the appendedclaims.

We claim:

1. A high temperature protective coating composition for metalconsisting essentially of parts by weight of a ceramic material, fromabout 10 to about 200 parts by weight aluminum, from about 1 to about100 parts by weight of a bentonite clay composition and an amount ofdiluent sufiicient to give the composition a spreadable consistency.

2. The composition of claim 1 containing in addition from about 1 toabout 100 parts by weight nickel powder.

3. The composition of claim 1 containing in addition from about 1 toabout 100 parts by weight nickel powder and from about 1 to about200-parts by weight of an organic binding composition.

4. The composition of claim 1 containing in addition from about 1 toabout 100 parts by weight of nickel powder and from about 1 to about 200parts by weight of an organic binding composition and wherein thediluent is selected from the class consisting of water and volatileorganic compounds in an amount of from about 1 to about 10 times thetotal solid content.

5. A high temperature protective coating composition for metalconsisting essentially of 100 parts by weight of a ceramic material,from about 50 to about parts by weight of aluminum, from about 2 toabout 50 parts by weight of a bentonite clay composition, and an amountof diluent sufficient to give the composition a spreadable consistency.

6. The composition of claim 5 containing in addition from about 2 toabout 50 parts by weight of nickel powder.

7. The composition of claim 5 containing in addition from about 2 toabout 50 parts by weight nickel powder and from about 50 to about 150parts by weight of a compatible organic binding composition.

8. A high temperature protective coating composition for metalsconsisting essentially of substantially 100 parts by weight of a ceramicmaterial, substantially 70 parts by weight aluminum flakes,substantially 10 parts by weight dimethyl didodecyl ammonium bentonite,sub stantially 10 parts by weight nickel powder, substantially 30 partsby weight of methyl methacrylate resin and substantially 290 parts byweight of methyl isobutyl ketone.

9. A process of heat-treating metallic articles comprising applying tothe surface of said article the high temperature coating composition ofclaim 1 to form a coated article, initially heating said coated articleto ture suflicient to fuse said coating, and thereafter heattreatingsaid metallic article at a predetermined temperature.

10. A process of heat-treating metallic articles comprising applying tothe surface of said articles the high temperature coating composition ofclaim 8, to form a coated article, initially heating said coated articleto a temperature sufficient to decompose the organic material of saidcomposition to form a film of non-combustible material on the surface ofsaid article, heating said film to a temperature sufiicient to fuse saidfilm and thereafter heat-treating said metallic article at apredetermined temperature.

11. The process of heat treating a metal article comprising applying tothe surface a high temperature coating composition of claim 3, initiallyheating said coated article to a temperature sufficient to decompose theorganic matter in said coating to form a film on the surface of saidarticle, and thereafter heat treating said metal article at apredetermined temperature.

12. The process of heat treating a metal article comprising applying tothe surface a high temperature coating composition of claim 8, initiallyheating said coated 13 article to a temperature suf'ncient to decomposethe organic material in said composition to form a film on the surfaceof said article and thereafter heat treating said metal article at apredetermined temperature.

13. The process of heat treating a metal article comprising applying tothe surface of said article the composition of claim I initially heatingsaid article to a temperature suificient to fuse said composition onsaid surface, and subsequently heat treating said article at atemperature of from about 1100 to about 1850 F. for a period of fromabout 15 to about 60 minutes, cooling said article and removing saidcoating from surface.

14. A process of heat treating titanium metals and a1- loys thereofcomprising coating said titanium with the composition of claim 8,heating said coated titanium articles to a temperature of substantially575 F. for a 14 period of substantially minutes and then heat treatingsaid article at a temperature of from about 1400 to about 1850 F. for aperiod of substantially 30 minutes, and quenching said heat treatedarticle.

15. A metal article having a coating consisting of essentially parts byweight of a ceramic material, from about 10 to about 200 parts aluminum,and from about 1 to about 100 parts of a bentonite clay composition.

16. A metallic article having a coating consisting essentially of 100parts by weight of a ceramically maturable vitreous enamel composition,from about 50 to about parts by weight of aluminum, from about 2 toabout 50 parts by weight of a bentonite clay composition, from about 2to about 50 parts by weight of nickel.

No references cited.

1. A HIGH TEMPERATURE PROTECTIVE COATING COMPOSITION FOR METALCONSISTING ESSENTIALLY OF 100 PARTS BY WEIGHT OF A CERAMIC MATERIAL,FROM ABOUT 10 TO ABOUT 200 PARTS BY WEIGHT ALUMINUM, FROM ABOUT 1 TOABOUT 100 PARTS BY WEIGHT OF A BENTONITE CLAY COMPOSITION AND AN AMOUNTOF DILUENT SUFFICIENT TO GIVE THE COMPOSITION A SPREADABLE CONSISTENCY.